The Active NBS Liege study is a monocentric, academic, fully remote, observational study designed to validate digital measures of motor development in children with spinal muscular atrophy (SMA) or Duchenne muscular dystrophy (DMD) identified through newborn screening, family testing, or incidental diagnosis. The study will enroll 100 children and follow them longitudinally for up to 30 months. Participants are remotely recruited, and all procedures, including consent, questionnaires, and follow-up visits, are conducted by phone or video conferencing without any hospital visits. Children will use age-appropriate wearable devices at home: MAIJU®, a sensorized garment for non-ambulant infants, and Syde®, an ankle-worn sensor for ambulant children. Data collection includes digital motor endpoints, clinical information, and quality of life (PedsQL). Primary objectives are to validate digital biomarkers of motor development, while secondary objectives include early identification of motor deficits, modeling motor trajectories, and quantifying genotype-related differences. Exploratory analyses will assess gait parameters such as stride velocity 95th centile (SV95C) and compare motor outcomes across genetic profiles and treatment exposure. Risks are minimal, limited to the use of non-invasive sensors with no known side effects.